Directional microphones offer advantages over omnidirectional microphones in noisy environments. Unlike omnidirectional microphones, directional microphones can discriminate against both solid-borne and air-borne noise based on the direction from which such noise emanates, defined with respect to a reference axis of the microphone. Differential microphones, sometimes referred to as gradient microphones, are a class of directional microphones which offer the additional advantage of being able to discriminate between sound which emanates close to the microphone and sound emanating at a distance. Since sound emanating at a distance is often classifiable as noise, differential microphones have use in the reduction of the deleterious effects of both off-axis and distant noise.
Differential microphones are microphones which have an output proportional to a difference in measured quantities. There are several types of differential microphones including pressure, velocity and displacement differential microphones. An exemplary pressure differential microphone may be formed by taking the difference of the output of two microphone sensors which measure sound pressure. Similarly, velocity and displacement differential microphones may be formed by taking the difference of the output of two microphone sensors which measure particle velocity and diaphragm displacement, respectively. Differential microphones may also be of the cardioid type, having characteristics of both velocity and pressure differential microphones.
As a general matter, differential microphones exhibit a frequency response which is a function of the distance between the microphone and the source of sound to be detected (e.g., speech). For example, when a pressure differential microphone is located in the near field of a speech source (that area of the sound field exhibiting a large spatial gradient and a large phase shift between acoustic pressure and particle velocity, e.g., less than 2 cm. from the source), its frequency response is essentially flat over some specified frequency range. At somewhat greater distances from the speech source, the frequency response tends to over-emphasize high frequencies. When a velocity differential microphone is in the near field of a speech source, its frequency response tends to over-emphasize low frequencies, while at somewhat greater distances, its response is essentially flat for some specified frequency range.
Because their frequency response varies with distance, differential microphones are ideally suited for use at a constant distance from a source, for example, at a distance where microphone response is flat. In practice, however, users of pressure differential microphones often vary the distance between microphone and mouth over time, causing the microphone to exhibit an undesirable, variable gain to certain frequencies present in speech. For a pressure differential microphone, unless a close constant distance is maintained, high frequencies present in speech will be emphasized. For a velocity differential microphone, unless somewhat greater distances are maintained, lower frequencies will be emphasized.